Cleanup and simplify the HLS Recorder.
MythDownloadManager has a memory leak, which can be terminal when used by

the HLS Recorder.  Add a simple MythSingleDownload class which uses
QNetworkAccessManager directly to avoid the leak, until it can be found and
fixed.
The classes defined in httplivestreambuffer.cpp were originally designed for

HLS playback, and are inefficient for recording.  Create a new
recorder-specific HLS processing class called HLSReader.  Since it is
specifically designed for recording, HLSReader is more efficient and faster.